Here are my results about Legendary/Rainbow hybrid ( Opponent Opal Octopus or Grand Unicorn):
(Use against Legendary/Rainbow hybrid) Strong: Dark; Egypt; Treasure; Tree
(Avoid using against Legendary/Rainbow hybrid) Weak: Autumn; Electric; Fire; Nature; Royal; Water
Neutrals with Legendary/Rainbow hybrid: Birthstone; Christmas; Earth; Fairy; Halloween; Ice-cream;Legendary (crystal); Lucky (Shamrock); Magic; Music; Rainbow; Realm Rune; Service; Space ( Planet); Spooky; Spring; Valentine; Winter; Zodiac
Currently untested with: Bomb; Food; Skull
(All tests are done on IOS device, based only on Strong/ weak / neutral hits. Critical hits have not been taken into account. )